1. Field of the Invention
The present invention relates to a control apparatus for a rectifying circuit adapted to improve the power factor and the waveform of an AC input current flowing through the rectifying circuit formed of a diode bridge.
2. Description of the Related Art
Generally, in order to simply realize a DC power source, there is used a rectifying circuit which obtains a DC power by rectifying an AC power through a diode bridge circuit and then smoothing it through a capacitor. An AC input current flowing through such a rectifying circuit is dependent on the difference between an AC input voltage of the circuit and the charged voltage of the smoothing capacitor. The capacitor is usually charged up to nearly the peak value of the AC input voltage of the diode bridge circuit, and hence the AC input current begins to flow only when the AC input voltage increases nearly to its peak value. Accordingly, the AC input current pulsates, having a greatly distorted waveform including low order harmonic components such as the third and the fifth harmonics. To improve such a waveform of the AC input current, it has been practice to provide a voltage boosting chopper circuit between the DC output side of the diode bridge circuit and the smoothing capacitor so that the waveform of the AC input current approaches a sine wave.
An apparatus as disclosed in the Japanese Patent Laid-open No. 59-198873 is known, for example. According to this, a signal corresponding to the AC input voltage of a diode bridge circuit is multiplied by the deviation between a constant reference voltage and the actual voltage of a smoothing capacitor to obtain a current reference signal synchronized with the AC input voltage, and a chopper circuit provided between the diode bridge circuit and the smoothing capacitor is so operated as to make the AC input curren follow the current reference signal to thereby improve the power factor and the waveform of the AC input current.
I such an apparatus, however, there are problems described below. Capacitance of the smoothing capacitor is usually as large as 2000 to 3000 .mu.F, in case the rectifying circuit is used as a DC power source for a voltage source type inverter, which supplies three phase AC power for an induction motor of an air conditioning appliance, a belt conveyor and so on. In such cases, the induction motor, and therefore the DC power source, must be able to cope with the widely varying load and to often bear the suddenly changing load, i.e. an impact load. However, since the smoothing capacitor is of large capacitance, the terminal voltage of the capacitor does not change unless the load varies to a considerable degree; especially in the low load region, it scarcely changes. As a result, the deviation signal between the constant reference voltage and the actual voltage of the smoothing capacitor is not produced, so that the copper circuit does not operate and hence the function of improving the power factor and the waveform of the AC input current fails to be attained.
In the case of the impact load, a load current increases rapidly, while the change in the voltage of the capacitor is delayed due to the large capacitance. This delay results in the delay of the control of the capactor voltage. Thereby the condition of the over current is temporarily brought about. Further, the delay of change in the capacitor voltage also causes the result that it takes more time until appearance of the desired deviation between the constant reference voltage and the capacitor voltage. Therefore, the effect of improving the power factor and the waveform of the AC input current can not be obtained sufficiently until the desired deviation signal appears.